The present invention relates to magnesium electrolysis cells and linings therefor which give an increased service life.
Magnesium is the eighth most abundant element in the earth's crust and the third most abundant element in sea water. There are two principal commercial processes to obtain magnesium, thermal and electrolytic, with the electrolytic process accounting for the vast percentage of commercial production.
In the electrolytic process, sea water is utilized as the source of the magnesium, with the the Dow electrolytic process being a well known procedure. In such electrolytic processes an electrolysis cell is utilized and magnesium chloride concentrated from sea water is separated into magnesium metal and chlorine gas. It is conventional and necessary to use refractories in such cells, particularly to line the upper sidewalls of the magnesium electrolysis cells, in order to contain the salt bath and metal entrained in the bath and to prevent corrosion of the steel shell. The term "upper sidewalls" refers to the molten metal electrolyte melt line and above. Below this line, no refractory is used since the steel sides and bottom of the electrolytic cell act as cathodes for the electrolysis process. Steel is an acceptable material for containment of molten magnesium. However, at the melt line, and above, chlorine gas and hydrochloric acid vapors are concentrated and could corrode the steel very quickly and easily which is why refractories are used in the "upper sidewalls" of the electrolysis cell. The magnesium metal and the molten salt bath contained within the cell are very fluid and, hence, readily wet the surface of refractories and can easily penetrate into any cracks, fissures, or porosity in the lining. Further, magnesium metal is also very reducing and can attack many of the oxides contained in refractories. In addition, the alkali chlorides used to make up the electrolyte bath can attack certain components of the refractories, particularly the fine-grain bonding matrices. All of these conditions, along with the circulation of the electrolyte bath within the cell, lead to significant amounts of corrosion of the refractory lining.
Furthermore, above the electrolyte bath the refractories are exposed to a reducing atmosphere containing chlorine gas and hydrochloric acid vapors from the electrolysis of the magnesium chloride feed, and also carbon monoxide and carbon dioxide from the oxidation of the graphite anodes used in the cells. Lastly, there is superheated water vapor from the dehydration of the hydrous magnesium chloride feed. These gases also readily penetrate into any open porosity and attack certain components of the refractory and its bonding matrix.
A number of different types of refractory materials have been tried to give the best corrosion resistance. Early on, hard-burned, low porosity, and low permeability fire clay brick were utilized in magnesium electrolysis cells and although they contain less open porosity than typical refractories, they still were unsatisfactory due to the fact that they were penetrated by magnesium metal, alkali chlorides, and gases from the reducing atmosphere contained in the electrolysis cells. Further, the alkali chlorides would attack the bonding matrix forming expansive alkali phases and soluble chloride phases and cause the hot face of the refractory to become weak and friable. This lead to further penetration through the disrupted region and the circulation of the electrolytic bath caused corrosion of the hot face.
Sintered, high alumina compositions were also attempted to be utilized, but it was found that they reacted with the electrolyte bath in a similar fashion as the fire clay refractories noted above. Further, they had an inherently higher open porosity than fire clay brick which made them even less satisfactory.
Efforts to utilize other refractory materials such as sintered, alumina-chrome solid solution, high alumina compositions and sintered magnesia brick were also tried but each was also found to be unsatisfactory. In the case of the alumina-chrome solid solution bonded high alumina chrome compositions, penetration of the electrolyte bath caused extensive reorganization of the bonding matrix due to the fact that the magnesium metal reduced it to metallic aluminum and chromium and, thus, no alumina-chrome solid solution bond remained. Also, the magnesia present reacted with additional alumina from the bonding matrix to form an expansive spinel phase, which weakened the refractory shape and made it susceptible to spalling.
With respect to the sintered magnesia brick, the fine magnesia of the bonding matrix was attacked by the chlorides in the electrolysis bath which weakened the brick. In addition, superheated water vapor from the dehydration of hydrous magnesium chloride feed caused hydration of the magnesia, resulting in formation of an expansive brucite phase which further weakened the brick.
Fused cast refractories were also tried including alumina, magnesia, mullite, and chromite based compositions. Although these fused cast compositions did show some improvement over the sintered refractories previously used, they still did not provide the desired surface life due to reaction with components of the electrolytic bath.
However, it was found that fused cast magnesium aluminate spinel compositions increased the service life of the refractory linings of magnesium electrolysis cells and refractories made therefrom have performed well in the upper side walls of magnesium electrolysis cells. However, despite their improvement over the prior refractories discussed above, they are still not satisfactory. Because of the manner in which fused cast refractories are formed, many types of imperfections occur during the manufacture thereof. A large volume shrinkage occurs upon cooling and crystallization of the melt results in casting voids within the shade. Gases dissolved in the melt are released during crystallization which can result fine porosity in the final shape. Varied crystal sizes, texture and composition can result due to different cooling rates experienced by the shape as cools from the exterior surface to the center. Moreover, if the shapes are not properly cooled and, particularly, if cooled too quickly, the stresses generated during crystallization do not have time to be adequately relieved, resulting in either cracking or very fine cracks in the formed refractory shape.
In addition to the disadvantages of fused cast refractories related to the manufacturing process there are inherent shortcomings in the final products themselves. First, thermal conductivity of the fused cast refractories is almost double that of sintered refractories and the heat losses from furnace linings are very significant. Further, the thermal expansion is also very high. Therefore, expansion allowances and thermal shock due to temperature fluctuations in the cell have to be taken into consideration during furnace design. Moreover, because of the problems inherent in forming fused cast refractories, their production is limited to simple shapes and these shapes are not easily cut or drilled. Thus, the manufacturing cost for fused cast refractories is very high due to processing requirements such as high electrical energy cost of melting the raw materials, the mold costs, and, as noted, the labor intensive finishing operations that are necessary to form the shapes.